Session 5: Precast Concrete Floors. Rick Henry Courtesy of University of Auckland, Faculty of Engineering

Transcription

1 Session 5: Precast Concrete Floors Rick Henry Courtesy of University of Auckland, Faculty of Engineering

2 Background Precast concrete floors widespread in NZ Many advantages with precast units Connections must be adequate to ensure: Diaphragm action (inertia + transfer) Gravity loads

3 Overview Review of diaphragm behaviour Covered at 2014 Strut and Tie seminars Deformation mechanisms at precast unit supports Design of support connections EQ performance and testing Best practice details

4 Diaphragm Behaviour [Des Bull 2014 slides]

5 Function and performance of floor diaphragms Transfer lateral inertial forces to vertical elements of the seismic force-resisting system. F i : floor, beams, columns and cladding EQ Inertia effects, distributed across the floor. C C C Note: the tie T is connected at the interior columns. T

6 Plastic Elongation of the beams push the columns Loss of floor supports and load paths.??

7 Case study Canterbury Earthquakes: Clarendon, 1987 Perimeter RC MRF

8 Red arrows: Beam elongate pushing the corner column away from the floor.

9 Cold drawn wire mesh fractures from 2 mm to 5 mm.

10 Loss of Strut load paths. Elongation causes the loss of the load path for the struts. Loss of Strut UoC Report 2010

11 Diaphragms: Connections or Nodes of the Struts and Ties Column-Beam Node: Traditional view Higher compressive stress - smaller contact surface

12 Diaphragms: Connections or Nodes Easiest visualisation: Intermediate column act as NODES of truss But middle of beams OK except for corner bays? of the Struts and Ties

13 Diaphragms: Connections or Nodes of the Struts and Ties Ties provided already as column restraint = 5% Axial load Or a % of shear in columns NZS 3101:2006 e.g. 4 - D16

14 Deformation Mechanisms in Floors

15 Earthquake Induced Actions in Multi-storey Buildings Structure conforms to beam sway profile Dominant actions at support connection: 1. Relative rotation induced between flooring units and support 2. Beam elongation in plastic hinges Jenson et al.

16 Relative Rotation Between precast unit and supporting beam Can develop flexural failure modes in floor unit (both positive and negative moments) After Jenson et al. Worst case = loss of support

17 Beam Elongation Often 2-4% of the beam depth Increases the distance between the unit supports Displacement incompatibilities Des Bull

18 NZS 3101 Support connections cl Seating details to accommodate rotation and elongation at peak interstorey drifts + 50% larger than ULS earthquake Seating length / detailing Precast unit design Support beam design Fenwick et al.

19 Precast Floor Unit Connection Details

20 Link Slabs Displacement incompatibility between the frame and h/c unit Hollow-core unit sags, (applies to any precast floor) Frame deforms in double curvature Des Bull Incompatibility with both single and double bay spans Either floor is designed to deform (+fixed connections), or, isolate floor and frame incompatibility

21 Link slabs: Matthews, Lindsay & MacPherson Timber in-fill 600 mm NZS 3101:2006 Cl Des Bull

22 Link slabs: Maintain struts Struts Ties W N S E Unit 4 Unit 3 Unit 2 Unit 1 Des Bull

23 Hollow-core Extruded prestressed section

24 Hollow-core CHCH Positive moment cracking Old detailing Credit: John Marshall Fenwick et al.

25 Hollow-core CHCH Displacement incompatibilities Resolved with modern detailing Credit: Rod Fulford Credit: John Marshall

26 Hollow-core CHCH Minor spalling of support ledge Newer details Credit: Rod Fulford Credit: Rod Fulford

27 Hollowcore Support Detail NZS 3101:2006 recommended detail: Prevent rotation damaging brittle hollowcore unit Sufficient seating length

28 Hollowcore Support Detail Test

29 Link to UC Research Report Assessment of hollow-core floors for seismic performance

30 Double Tees Flange hung or web supported / dapped end High bearing stresses

31 Double Tees CHCH Credit: Rod Fulford and John Marshall

32 Double Tees Support detailing Web supported: Armoured (SESOC) Flange hung: Robust hanger

33 Rib and Timber Infill

34 Rib and Timber Infill Rib sometimes cast into support Concerns with positive moment cracking Credit: Des Bull

35 Rib and Timber Infill Positive moment cracking Fenwick et al. Credit: John Marshall Credit: John Marshall

36 Rib and Timber test

37 Rib and Timber Infill Support detailing SESOC

38 Rib and Timber test

39 NZS 3101 New provisions in A3 for floor seating, when: Interstorey drift > 0.6% or elongation >10mm Seating width calculated, including: Bearing MCE MCE Loss of cover (unit + support) Shrinkage Tolerances

40 NZS 3101 Support beam ledge: Bearing design (corbel) Armouring when necessary Recommended support details shown in commentary

41 Design Example [Additional notes]

42 Seating Length Example 100 mm 300HC 1000? Design insterstorey drift of 2% 600

43 Seating Length Example 100 mm 300HC 1000 Elongation of parallel hinge (one hinge per end) = peak ULS drift x 1.5 (max = 0.036h b ) = x 1000 = 36 mm 600

44 Seating Length Example 100 mm 300HC Elongation is measured at the beam centreline. Additional elongation can occur at the support ledge due to the support beam rotation. 600 Rotation of support beam reducing support: = (peak ULS rotation x 1.5) x height to ledge = (0.02/0.7*1.5) x 100 mm = 5 mm

45 Seating Length Example 100 mm 300HC 1000 Spalling at back of unit = 20 mm (unarmoured) Spalling of support ledge = face to centerline of longitudinal bars (when in plastic hinge region) = cover (30) + stirrup (10) + ½ bar (10) = 50 mm 600

46 NZS 3101 A3 Seating width calculated, including: Bearing = 10 mm (check bearing stress) MCE = 36 mm (0.036*1000 mm parallel beam) MCE = 5 mm (0.02/0.7* mm) Loss of cover (unit + support) = 20 mm + 50 mm Shrinkage = 2 mm (assume 0.5mm/m) Tolerances = 17 mm Total = = 140 mm Extreme case: Would be less for lower drifts, or could be reduced by armouring ledge

47 Summary Floors need to accommodate deformations including rotations and elongations in ductile multi-storey buildings Extensive research on recommended connection detailing Preventing damage to floor unit Seating width for larger than ULS earthquake Robust support detailing

48 Questions?